That question must have just been devastating to Peter when Jesus asked it.  Only a few days earlier Peter had denied knowing Jesus three times!  And that came after a promise was made by Peter to Jesus to never leave His side.  Now Peter was confronted by the One he had followed for three years: do you love me?  Of course Peter loved Jesus.  But then he was asked again: Peter, do you love me?  Perhaps Peter was uncertain of why Jesus asked again.  Or perhaps Peter was feeling convicted for the shame he felt as a result of the denial of his friend.  Whatever the case, Peter again affirmed his love for Jesus.  For a third time Jesus posed the question: do you love me?  Peter replied differently this time: "Lord, you know all things; you know that I love you."  Peter apparently needed affirmation that his lapse of judgment a few days prior was not indicative of his devotion to Jesus.  Jesus knew that Peter loved Him, but did Peter know that he loved Jesus?  This affirmation set a fire in Peter than enabled him to lead the start of the Church.  Peter failed, but he came back.  He made a mistake, but he was reminded that he was still on Jesus' team.  I think we need that reminder sometimes.  If you've fallen down, get back up.  Your lapse of judgment isn't necessarily indicative of your whole person.  Stand up, and move forward; Jesus may have some very important work for you to do that is not possible to complete if you're still on the ground.